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Pro Helps deal with duplicates across multiple accounts
Including Google, iCloud, Microsoft Exchange, Twitter, Foursquare, Xing and Linkedin accounts.
Pro Great integration with Google's services
FullContact pulls information about contacts from social networks to give an overview of the person right from within Gmail. It shows information about meeting attendees in Google Calendar. It supplements Google Contacts by adding additional information to entries.
Pro Keeps information about contacts up to date
Free version synchronizes contact information daily while the premium version allows real-time sync.
Pro Contacts can be organized via tags
Pro Accurate business card transcription
FullContact uses human transcription that allows for near-perfect accuracy.
Pro Seamless translation between carddav (Google, Apple) and .vcf (Outlook)
Apple/Google/Yahoo employ a similar format type of contact card called cardDAV/webDAV while Outlook uses a .vcf format extension.
Anyone who has tried to sync Google to Apple or vice versa knows that despite a similar format, just getting those two to sync well without losing details or mashing up data can be very hard. FullContac seamlessly integrates the address books of all of the major players and does so really well.
Pro Allows leaving notes for each user
Con Advanced features require payment
It's subscription-based and there's a monthly cost depending on the feature tier you choose.